. Fibrillar Aβ causes profound microglial metabolic perturbations in a novel APP knock-in mouse model. bioRxiv. January 20, 2021.

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  1. This is a mouse model for amyloidosis, which, finally, is freely available for the entire research community. This is a big issue, considering that many experiments were blocked or slowed due to legal issues with mouse models. I think this is a great sign for our research community to freely interact and to finally stop the very unproductive or even counterproductive Materials Transfer Agreement business. After all, our mission is primarily to provide the best data and to translate them as soon as possible to preclinical and clinical research. Many thanks to the scientists at Denali for making this possible!

    We are using this model to further test the preclinical properties of our TREM2 modulating antibody 4D9.

    There are as always still open questions. Due to the unique combination of three mutations, this model has its limitations. In vitro data suggest that the Arctic mutation used in this model could drive Aβ38 production, which is a bit puzzling, since that species may not be deposited and is thought be produced at the expense of Aβ42. One may also have to consider that some antibodies could fail to recognize amyloid deposits, due to the potential structural changes caused by that mutation. 

    View all comments by Christian Haass
  2. Xia et al. present a new APP knock-in (KI) mouse model for Aβ amyloidosis that expresses humanized Aβ with three familial, early onset AD mutations and recapitulates the spatial patterning of plaque deposition observed in humans under physiological conditions. The homozygous KI mice accumulate appreciable parenchymal plaques and leptomeningeal vascular deposits (CAA) after 8 months of age, which makes this model practical enough for use in basic and preclinical studies. 

    I applaud Denali for making this model accessible to all—something the AD research community should be trying to reconcile for existing animal models, and striving toward in the release of future ones. One consideration, however, is that the E22G Aβ (E693G APP) “Arctic” mutation will produce a distinct conformational variant of Aβ in these mice that does not reflect the predominant conformation(s) formed by wild-type Aβ in sporadic, late-onset AD patients. 

    Using fluorescent structure-sensitive dyes in brain samples from human subjects bearing the very rare E22G mutation, we have shown that E22G Aβ deposits have a unique conformation that does not overlap much with wild-type Aβ deposits in sporadic AD; similarly, synthetic E22G Aβ fibrils formed in vitro exhibit a distinct dye-emission signature compared to wild-type (Condello et al., 2018). 

    Moreover, the Aβ isoform composition of plaques is distinguishable in E22G mutation carriers compared to wild-type Aβ in sporadic AD. For example, the Aβ38 isoform is not commonly found in amyloid deposits, but in intra-Aβ mutation carriers (e.g., also in E22Q “Dutch” and D23N “Iowa” cases) this seems to be a hallmark feature (Moro et al., 2012). Notably, using an inoculation paradigm in susceptible wild-type Aβ mice, we demonstrated that injecting synthetic E22G Aβ fibrils or patient brain extract from an E22G mutation carrier induced a distinct plaque conformation with increased Aβ38 deposition (Condello et al., 2018; Watts et al., 2014), which argues that the conformation of E22G deposits more efficiently template the Aβ38 isoform. 

    This new KI model also bears the “Austrian” mutation near a γ-secretase cleavage site, and familial mutations in this region have been reported to increase Aβ38 production (Suárez-Calvet et al., 2013). While it remains unclear how the Aβ38 isoform contributes to pathogenicity, it would be interesting to know if this new KI model also exhibits such molecular features. Interestingly (at least based on the images displayed in the preprint), Xia et al. show Aβ plaques that appear more filamentous or diffuse in shape, resembling the morphology described for the E22G familial AD human cases (Kalimo et al., 2013). 

    As the evidence grows for Aβ conformational heterogeneity within and between AD patients and etiologies (Cohen et al., 2015; Qiang et al., 2017; Rasmussen et al., 2017; Condello et al., 2018), there is an emerging hypothesis that distinct conformational variants may underlie different phenotypic manifestations of AD. So, perhaps we should no longer generalize that a plaque is a plaque—it’s not all the same, at least at the molecular level. Thus, it seems reasonable to question if the plaques composed of mutant Aβ in this new KI model are representative of those in sporadic AD and lead to similar consequent pathobiological pathways found in mouse models or humans producing wild-type Aβ. 

    Beyond the histological validation of several plaque-associated features such as microglial activation and dystrophic neurites, Xia et al. present RNA-sequencing data from sorted microglia, and show that there is a set of differentially expressed disease-associated (DAM) and plaque-induced (PIG) microglial genes that partially overlaps with DEGs reported in the popular 5xFAD overexpression model. This supports the notion that there is a generalized microglial transcriptional programming toward extracellular and/or phagocytosed Aβ.

    It would be fascinating to know if the non-overlapping microglial DEGs in this new APP KI model manifest because of unique signaling induced by mutant Aβ, or the failure to degrade it. Curiously, histological studies have observed a muted plaque-associated microglial phenotype in E22G familial AD brain samples compared to sporadic AD (Kalimo et al., 2013). Because we now appreciate that microglia are heterogeneous cells (Stratoulias et al., 2019) that elicit specialized responses to different pathologies (Friedman et al., 2018), it is plausible that distinct microglia function (and dysfunction) occurs because of Aβ plaque variants. If true, this has great implications for the discovery and development of targeted molecular therapies.

    References:

    . Structural heterogeneity and intersubject variability of Aβ in familial and sporadic Alzheimer's disease. Proc Natl Acad Sci U S A. 2018 Jan 23;115(4):E782-E791. Epub 2018 Jan 8 PubMed.

    . APP mutations in the Aβ coding region are associated with abundant cerebral deposition of Aβ38. Acta Neuropathol. 2012 Dec;124(6):809-21. PubMed.

    . Structural heterogeneity and intersubject variability of Aβ in familial and sporadic Alzheimer's disease. Proc Natl Acad Sci U S A. 2018 Jan 23;115(4):E782-E791. Epub 2018 Jan 8 PubMed.

    . Serial propagation of distinct strains of Aβ prions from Alzheimer's disease patients. Proc Natl Acad Sci U S A. 2014 Jul 15;111(28):10323-8. Epub 2014 Jun 30 PubMed.

    . Autosomal-dominant Alzheimer's disease mutations at the same codon of Amyloid Precursor Protein differentially alter Aβ production. J Neurochem. 2013 Oct 11; PubMed.

    . The Arctic AβPP mutation leads to Alzheimer's disease pathology with highly variable topographic deposition of differentially truncated Aβ. Acta Neuropathol Commun. 2013 Sep 10;1(1):60. PubMed.

    . Rapidly progressive Alzheimer's disease features distinct structures of amyloid-β. Brain. 2015 Apr;138(Pt 4):1009-22. Epub 2015 Feb 15 PubMed.

    . Structural variation in amyloid-β fibrils from Alzheimer's disease clinical subtypes. Nature. 2017 Jan 12;541(7636):217-221. Epub 2017 Jan 4 PubMed.

    . Amyloid polymorphisms constitute distinct clouds of conformational variants in different etiological subtypes of Alzheimer's disease. Proc Natl Acad Sci U S A. 2017 Dec 5;114(49):13018-13023. Epub 2017 Nov 20 PubMed.

    . Structural heterogeneity and intersubject variability of Aβ in familial and sporadic Alzheimer's disease. Proc Natl Acad Sci U S A. 2018 Jan 23;115(4):E782-E791. Epub 2018 Jan 8 PubMed.

    . The Arctic AβPP mutation leads to Alzheimer's disease pathology with highly variable topographic deposition of differentially truncated Aβ. Acta Neuropathol Commun. 2013 Sep 10;1(1):60. PubMed.

    . Microglial subtypes: diversity within the microglial community. EMBO J. 2019 Sep 2;38(17):e101997. Epub 2019 Aug 2 PubMed.

    . Diverse Brain Myeloid Expression Profiles Reveal Distinct Microglial Activation States and Aspects of Alzheimer's Disease Not Evident in Mouse Models. Cell Rep. 2018 Jan 16;22(3):832-847. PubMed.

    View all comments by Carlo Condello
  3. The new knock-in mice described by Xia and colleagues carry Swedish (NL), Arctic (G) and Austrian (I) mutations (NL-G-I mice) are conceptually identical to our mice that carry Swedish, Arctic, and Beyreuther/Iberian (F) mutations (NL-G-F mice) (Saito et al., 2014). 

    The Swedish mutations increase the total amount of Aβ; the Arctic mutation renders Aβ oligomerization-prone and resistant to degradation by the major Aβ-degrading enzyme, neprilysin (Iwata et al., 2001; Tsubuki et al., 2003); the Austrian or Beyreuther/Iberian mutation increases the ratio of Aβ42/Aβ40 production. As a result, their pathological phenotypes, including microglial responses, appear similar. So many papers have already been published using our mice, and, for instance, the following two appear very important: Chen et al., 2020, and Sobue et al., 2021). 

    The mutant mice, patented in Japan, the United States, and EU, are available to industry and pharma after a license contract has been signed. More than 10 companies worldwide are using our mice. The license fee depends on the users’ financial situation. Here are some advantages of using our knock-in mice, although researchers are free to use any model.

    1. Our knock-in mice are available from RIKEN BioResource Center, a Japanese counterpart of Jackson Laboratory: RIKEN BioResource Research Center.

    2. Approximately 500 groups worldwide use our knock-in mice as we started distribution of the mice before publication, so experiments performed by different researchers can be compared in a relatively unbiased manner. Search PubMed for “Saido T” to identify published papers after 2014.

    3. Other line-ups are also available: NL and NL-F lines. The NL-F line shows less aggressive pathology than NL-G-F line but can be used to analyze how Aβ is anabolized and catabolized without the interference of the Arctic mutation.

    4. Human tau (hMAPT) knock-in mice in which the entire murine MAPT gene is humanized (Hashimoto et al., 2019; Saito et al., 2019), and double knock-in mice, i.e., NL-G-F X hMAPT and NL-F X hMAPT, are also from RIKEN BRC.

    5. Wild-type humanized Aβ mice and G-F mice will soon be made available. G-F mice, which accumulate pathological Arctic Aβ, can be used to characterize β-secretase and β-secretase inhibitors without the interference of the Swedish mutation.

    References:

    . Single App knock-in mouse models of Alzheimer's disease. Nat Neurosci. 2014 May;17(5):661-3. Epub 2014 Apr 13 PubMed.

    . Metabolic regulation of brain Abeta by neprilysin. Science. 2001 May 25;292(5521):1550-2. PubMed.

    . Dutch, Flemish, Italian, and Arctic mutations of APP and resistance of Abeta to physiologically relevant proteolytic degradation. Lancet. 2003 Jun 7;361(9373):1957-8. PubMed.

    . Dutch, Flemish, Italian, and Arctic mutations of APP and resistance of Abeta to physiologically relevant proteolytic degradation. Lancet. 2003 Jun 7;361(9373):1957-8. PubMed.

    . Spatial Transcriptomics and In Situ Sequencing to Study Alzheimer's Disease. Cell. 2020 Aug 20;182(4):976-991.e19. Epub 2020 Jul 22 PubMed.

    . Microglial gene signature reveals loss of homeostatic microglia associated with neurodegeneration of Alzheimer's disease. Acta Neuropathol Commun. 2021 Jan 5;9(1):1. PubMed.

    . Tau binding protein CAPON induces tau aggregation and neurodegeneration. Nat Commun. 2019 Jun 3;10(1):2394. PubMed.

    . Humanization of the entire murine Mapt gene provides a murine model of pathological human tau propagation. J Biol Chem. 2019 Aug 23;294(34):12754-12765. Epub 2019 Jul 4 PubMed.

    View all comments by Takaomi Saido

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